libpayload/string: add strndup() function

Change-Id: Ie509e49f21fb537692704ac6527efa09649164e3
Signed-off-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70115
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/string.h

@@ -59,6 +59,7 @@ char *strcat(char *d, const char *s);
 char *strchr(const char *s, int c);
 char *strrchr(const char *s, int c);
 char *strdup(const char *s);
+char *strndup(const char *s, size_t size);
 char *strstr(const char *h, const char *n);
 char *strsep(char **stringp, const char *delim);
 size_t strspn(const char *s, const char *a);